phổ 13C-NMR của Coumarins
Atta-ur-Rahman (Ed.) Studies in Natural Products Chemistry, Vol 18 © 1996 Elsevier Science B.V All rights reserved 971 C-NMR Spectroscopy of Coumarins and their Derivatives : A Comprehensive Review B Mikhova and Helmut Duddeck INTRODUCTION Coumarins constitute an important class in the realm of natural products with significant biolo- gical activity (1) Although many books and articles have appeared since 1970 containing ^^C NMR data of various classes of natural products, only a very few of them deal with coumarins derivatives, and these mainly cover the literature of the 1970s only (2,3) Since that time, however, the data of many more coumarins have been published and NMR spectroscopy has seen a revolution Thus, we believe that it is time to update the earlier reviews Since the eariy 1970s, ^^C NMR spectroscopy has developed into one of the most valuable tools for structure elucidation of organic compounds and natural products because the ^^c NMR spectrum is a fmgerprint of a given compound Moreover, ^^c data of a derivative not yet reported can often be extrapolated from the chemical shifts of compounds with related structural features Nevertheless, it is still mandatory to perform a safe ^^C signal assignment of unknown molecules in order to avoid misinterpretations which may lead to erroneous conclusions Therefore, we present a brief overview of NMR methods (section 2.1) which can be divided into two parts: a) The classical procedures have already been summarized by us before (3); nevertheless we include some of them here for the sake of completeness, b) New one- and two-dimensional NMR experiments have been designed during the 1980s which make some of the classical methods obsolete The data in this review have been compiled in a data base using MDL ISIS-Base Literature has been covered until spring 1995 972 METHODS OF i^C SIGNAL ASSIGNMENTS In general, ^^C NMR spectra are recorded under proton broad-band decoupling in order to avoid the severe signal overlap which can easily occur because of the large one-bond carbonhydrogen coupling constants ^J(C,H) = 120-250 Hz This procedure results in a breakdown of all signal splittings due to such couphngs Owing to the low natural abundance of the ^^C isotope (ca 1.1%), ^-^C NMR signals appear as narrow singlets if no further NMR-active nuclei with high natural abundance (e.g ^^F or ^ip) are present in the molecule This spectral simplification, however, produces a serious drawback in signal assignments since valuable coupling information is destroyed Thus, a variety of assignment methods have been developed some of which are introduced in this chapter There are five main areas: experimental NMR techniques, coupling constants, solvent effects, presence of auxiliaries and derivatization 2.1 Experimental NMR Techniques In the 1970s these techniques consisted mainly of ^H decoupling methods, the most prominent ones of which were the ^H broad-band (BB) decoupling and the single-frequency off-resonance decoupling (SFORD) techniques In SFORD, the decoupler frequency is positioned outside the ^H resonance range (off-resonance) Thus, all carbon-hydrogen couplings are reduced to such an extent that only the largest coupling constants, namely ^J(C,H), give rise to small residual splittings, from which the number of hydrogens adjacent to the respective carbon atoms can be read directly; singlets correspond to quaternary carbons, doublets to methine, triplets to methylene and quartets to methyl groups Nevertheless, in these spectra, signal overlap and second-order effects sometimes prohibit unambiguous interpretations in unfavourable cases After 1980, NMR spectroscopy saw a revolution due to the introduction of new one- and twodimensional experiments, new superconducting magnets providing magnetic fields up to 17.6 T (^Hresonance frequency: 750 MHz) and an enormous progress in computer technology New techniques, such as the recording of J-coupled spin echoes (Attached Proton Test), and INEPT and DEPT have been developed to circumvent these difficulties (4) By executing INEPT or DEPT involving polarization transfer from ^H to ^^C, the information about the number of adjacent hydrogens is also no longer reflected in residual signal splittings as in SFORD but in signal phases and intensities; CH and CH3 signals appear as positive and those of CH2 as negative singlets (Fig Ic) Alternatively, it is possible to suppress all signals except those of CH (Fig lb) Therefore, a comparison of these two DEPT spectra with the ^H BB-decoupled spectrum (Fig la) allows an unambiguous assignment of all four sorts of CHn fragments (n = 0-3) 973 9-0-^0 b •'-'^ i II*' i>A hi»M' wi*i